The present invention relates to a multiplex transmission system for vehicles that consists of a plurality of multiplex communication nodes which send and receive, by means of multiplex communication, information necessary for various kinds of electrical equipment related to numerous types of switches, meters or a steering provided in a vehicle. The present invention relates to the detection of the presence of a line disconnection of a multiplex transmission path formed in the form of a loop (i.e. a ring) as well as the locations of the above-mentioned disconnection and a defective node.
Regarding a network topology in a multiplex transmission system of this type for vehicles, various types of configurations, such as loop (ring) connection, bus connection, and star connection have been previously proposed. Each topology has its own features. For example, in the Japanese Patent Laid-Open No. 62-4658, a multiplex transmission path is made in the form of a loop (a ring). The reason for this is that if a line disconnection occurs at merely one place in the transmission path in a bus connection or a star connection, a specific node breaks down. On the other hand, making the transmission path in the form of a loop or a ring insures that a signal can be sent and received even if this transmission path is disconnected at one place.
In the above-mentioned loop or ring network, however, although signal transmission is possible even in a state with one line disconnection, it is difficult to know that it has occurred, because signal transmission is performed normally. Also, in a loop or ring network, where a disconnection occurs in a part of the transmission path, some degree of communication is insured. However, since the impedance of the line path is changed, noise resistance capability is decreased. Therefore, it is important to know that the line disconnection has occurred.
In a network such as a general LAN (Local Area Network), if a disconnection of a transmission path occurs, the entire system will not break down because of the breaking down of some terminals. However, in a multiplex communication system for vehicles, for example, if a node for meters breaks down due to a line disconnection, because the meter has become inoperable, the automobile itself cannot be driven as no alternatives for the meter node are available. Hence, if the presence of a line disconnection at one place is left unfixed and then another line disconnection occurs at another place, or if a transmission error occurs intermittently due to noise, this will lead to a complete breakdown of the vehicle.
Therefore, the present technology, as in the above-mentioned prior art, is not capable of insuring that a vehicle can be driven even in the case of a line disconnection and thus this technology cannot satisfy the requirements for the present-day multiplex communication system for vehicles. In order to meet the requirement, it is necessary at least to be able inform a driver or a service man at a service station of the presence of a line disconnection or the like before the vehicle breaks down, and further, it is important to easily locate the position of the line disconnection and defective nodes.